End users of voice over WiFi systems may experience poor audio quality or even a dropped call when using a handheld device that is unable to receive and decode voice and control packets on a noisy channel. For example, the periodic intermittent interference (PII) emitted by microwave ovens may disrupt or impair communications between an access point and a handheld device implemented according to the IEEE 802.11 standard (and operating in the 2.4 GHz band) and the access point with which it is associated to obtain network connectivity. Household microwave ovens use a magnetron tube that generally operates with about a 50% duty cycle over 16.67 ms ( 1/60 Hz) periods in North America (20 ms, 1/50 Hz in the rest of the world). In other words, the timing characteristics of the emitted energy can generally be described as a square wave, cycling on for about 8.33 ms, then off for about 8.33 ms (on/off for about 10 ms in the rest of the world). A means of avoiding on-channel interference from other devices is desired to improve the audio quality experience of the end users who own and operate voice over WiFi services.
Consider a Voice over WLAN (VoWLAN) handset using the access methods defined by IEEE 802.11. The access methods utilize a collision sensing mechanism referred to as the clear channel assessment (CCA) algorithm that senses the energy level on the channel. Since this algorithm is used to defer channel access while energy is above some threshold, the existing scheme may already seem well-suited to sensing microwave interference. Detection performance can, however, still suffer in certain scenarios. A major shortfall of this scheme is when the interference level is below the CCA threshold (particularly at one end of the link) but still high enough to result in a low C/I, and hence lost or corrupted packets, at one or both ends of the link.
With 802.11a/g, the CCA threshold is −62 dBm for an invalid Orthogonal Frequency Division Multiplexing (OFDM) frame, 20 dB higher than that for a valid frame, so this scenario is much more likely to occur with periodic intermittent interference (PII) than with another 11a/g device. When this scenario occurs, the CCA algorithm signals that the channel is clear and frame deliveries (both payload and any subsequent acknowledgements (ACKs)) are attempted while interference is present. The result is that initial delivery attempts are not likely to be successful. Retransmissions can occur based on the 802.11 distributed coordination function (DCF) mechanism which specifies exponentially longer maximum back-off lengths. However, since these are maximum lengths, there is the possibility that all retransmissions are attempted during a PII on-cycle. When this occurs, the frame is dropped. For VoWLAN applications, retransmissions are usually initiated at the MAC layer, and are usually limited to 6 retransmissions or so. The back-off periods between retransmissions are randomly drawn between zero and the contention window lengths that are specified for each retry in the standard. Taking into account the IEEE 802.11 Distributed Coordination Function, the cumulative back-off time is uniformly distributed between 0.238 ms and 27.5 ms for an initial delivery attempt followed by up to 6 retransmissions. So if a microwave oven is not detected (by the CCA algorithm), and an initial packet delivery attempt occurs during the magnetron tube's on-cycle, all retransmission attempts can occur (and fail) before the on-cycle ends.
Even when the CCA algorithm is successful at detecting PII on the channel, the channel may be sensed as being clear during a brief absence of PII only to have the interference return and corrupt the subsequent transmissions (data and/or ACKs) that are initiated. This is especially an issue for rapidly varying PII (relative to frame lengths).
Several published patent applications propose various means for avoiding transmitting on those frequencies/channels where intermittent interference is detected. None of these publications teach how to implement a detection mechanism, but a common theme is that each relies on knowledge of electromagnetic characteristics of the PII such as on/off timing and center-frequency. Even if detection is successful, some PII sources (such as certain microwave ovens) emit interference with significant spectral content across all WLAN channels. In this case there is no “clear” channel available to select.